The Tully Monster Mystery
The Tully Monster is a mysterious 307myr-old marine animal known only from the famous Mazon Creek fossil locality in Illinois. Its body plan is unlike any other animal that has ever lived, and it has been subject to wildly different interpretations as to its identity since its discovery in 1955. Last year, Victoria McCoy of Yale University and colleagues identified it as a lamprey, a primitive type of fish, but this has since been challenged by a team of vertebrate palaeontologists.
Fossil collector Francis Tully knew he had made an extraordinary discovery. Inside a rounded nodule was a bizarre, foot-long animal with a long trunk and claw. But he could never have known quite how extraordinary his 307myr-old fossil would turn out to be. Sixty two years later, scientists are still arguing over the basics as to what sort of creature it really was.
What makes it even stranger is that this is no rarity known only from fragmentary remains. After Tully made his find, word got around among collectors and, before long, hundreds more had been found. Tullimonstrum gregarium, or ‘Tully’s common monster’, is now known from well over a thousand fossils, including many complete specimens. “We’ve got four cabinets of Tully monsters here, each of which has 25 drawers,” says Paul Mayer of the Field Museum in Chicago, where the largest collection is held.
Common, well-preserved and complete fossils should be the very easiest to identify. So why has this one remained so enigmatic? And how did it acquire such notoriety that the MailOnLine recently suggested it was the ‘weirdest animal that ever lived’?
Tully, a pipe-fitter by trade, discovered the fossil in a pile of nodules discarded from a coal mine in the Mazon Creek area of Illinois. “One day in the summer of 1955 I found two rocks that had cracked open due to natural weathering. They held something completely different. I knew right away, I’d never seen anything like it. None of the books had it. I’d never seen it in museums or at rock clubs. So I brought it to Chicago to the Field Museum to see if they could figure out what the devil it was,” said Tully in an interview with the Chicago Tribune in 1987.
Weathering had split the nodule to reveal a creature with an elongated squid-like body and tail fin that was fronted by a long thin snout with a toothed claw at the end. Widely spaced eyes projected from a pole-like structure that stuck out on either side of its body. This body plan is different from any other creature that has ever lived.
Tully showed his fossil to Eugene Richardson, the curator of fossil invertebrates at the Field Museum, who was also baffled by the find. Fossil-rich ironstone nodules were found in shale beds that lay on top of coal deposits. Miners would move the shale and nodules aside to get at the coal which allowed collectors to scour the spoil heaps for fossils. Before long, hundreds more specimens had come to Richardson’s attention, but he was still none the wiser as to what it was.
Measuring between 8cm and 40cm long, the Tully monster has only ever been found at sites around the Mazon Creek, a tributary of the Illinois River, in north-eastern Illinois. The Mazon Creek is world famous for its abundant fossils of soft-bodied animals, which are not normally preserved. The site provides palaeontologists with the most complete snapshot known of the creatures that lived during the Carboniferous period of 359 to 299mya.
The Tully monster lived in a large estuary alongside jellyfish, sea anemones, a wide variety of marine worms, different types of mollusc, shrimps, horseshoe crabs and other arthropods, as well as several kinds of fish – lungfish, ray-finned and spiny-jawed fishes, coelacanths and sharks. Leaves and branches of land plants were also washed by a great river into the estuary, where it flowed into a shallow sea.
On land, swampy tropical forests dominated by giant clubmoss trees, seed ferns, horsetails and true ferns were home to amphibians and a rich variety of insects and other arthropods. Waterlogged soil in these forests created low oxygen conditions that inhibited the decay of dead trees and plants, which eventually resulted in the coal seams that were mined in modern times. The climate was hot and sticky and the site was located just ten degrees from the equator.
The fossils were formed when a colossal flood dumped huge amounts of sediment into the river delta. Animals living on the sea floor were smothered, while swimming creatures were jumbled up with plants and insects from land in a muddy grave. Very low levels of oxygen meant the buried organisms were not subject to rapid decay. Instead, bacteria slowly decomposed the remains, while emitting methane and ammonia, and triggered a series of chemical reactions that eventually resulted in nodules forming around the fossil nuclei.
Most Tully monster fossils came from Pit Eleven of the Peabody Coal Company strip mine near Essex in Illinois, located 105km southwest of Chicago. Nodules preserve the soft-bodied invertebrates as well as easier-to-preserve shells, fish skeletons, arthropod exoskeletons and plant leaves and twigs.
In 1966, Richardson named the ‘worm-like’ fossil in honour of its discoverer. Richardson had called it a “monster” when he first saw it and that description also made it into its Latin name. In 1969, he co-authored a paper that concluded the Tully monster “represents an extinct and previously unknown phylum”. In other words, it was so different from, for example, all known arthropods, molluscs, annelid worms and chordates (animals with cartilaginous skeletal rods, including those with backbones – the vertebrates), each of which are separate animal phyla containing a huge variety of species, that it deserved to be placed all by itself in a new phylum. It was “so strange as to be a biological orphan,” wrote Richardson.
More down to earth was Merrill Foster, of Bradley University in Illinois, who in 1979 concluded that it was an “aberrant member” of the molluscs. He compared it with sea snails, known as heteropod gastropods, that are found in tropical seas where they swim around searching for snails and soft-bodied prey to eat. Popularly known as ‘sea elephants’, owing to their trunk-like proboscises, heteropods arguably bear the closest resemblance to the Tully monster among living animals. However, they only have a fossil record that goes back to the Jurassic period, of 201 to 145mya, and Foster argued that the Tully monster was a sea snail that lived in a similar way to modern heteropods, but was not their direct ancestor. “I am regretfully aware of the fact that I may be removing part of the reason for the Tully Monster’s appeal,” he wrote.
He needn’t have worried. Subsequent studies brought its identity firmly back into question. A conference was held in 1989, at the University of Camerino in Italy, where two new papers were presented. Bret Beall, of the Field Museum, analysed 21 anatomical characters of the Tully monster in a cladistic analysis involving a variety of mostly invertebrate groups. He concluded that it was closest either to the conodonts (extinct, elaborately toothed, worm-like animals) or to heteropod-like sea snails, as argued by Foster. The evidence pointed either way. “After my paper was published, I continued my analysis and started to favour the chordates over the gastropods,” Beall told me in 2014, when I started work on this feature. Muddying the waters further was a cladistic analysis, by Frederick Schram of the Scripps Institution of Oceanography in La Jolla, California, that found that the Tully monster was most closely related to one or other of two unrelated types of worm: the annelid worms (of which earthworms are the best-known example), or the ribbon worms.
The Tully monster became the official state fossil for Illinois in 1989 and its fame continued to grow. It decorated the side of U-Haul trucks and trailers, and artists made models and cartoons of it. There was even an elaborate hoax in which Richardson was fooled by a mischievous former colleague into thinking that it still lived in a lake in Kenya.
University of Edinburgh dinosaur expert Steve Brusatte was born and raised in Illinois – it was coal mining that brought his family there in the first place. “It was this great mystery I learned about as a kid. It always stuck with me that there was this one of a kind fossil that was found in my backyard, and that scientists from around the world had come to collect and study, and that nobody knew what it was. That inspired me that there were these huge mysteries about the evolution of life that needed to be solved,” says Brusatte.
For a quarter of a century, that was how science left the Tully monster. It could be one of two types of worm, or a very unusual sea snail, or it could even be related to animals with backbones. Or it could be a completely new and unique type of animal.
I became interested in it in the 1990s, when I came across an illustration imagining how it looked in life. One glance at this alien-looking creature had me hooked. At that time, evolutionary biologist Stephen Jay Gould speculated in his book, Wonderful Life, that the ancient oceans were full of weird and wonderful animal ‘experiments’ that belonged to a wide range of extinct animal phyla, giving as examples the 508myr-old animals from the Burgess Shale in Canada. The ‘Cambrian Explosion’ had burst into life 542mya, spewing out unique body plans like confetti, argued Gould. The animal phyla that survived the Cambrian, which ended 485mya, were but a pale shadow of the diversity that had gone before. Surely, I thought, the Tully monster was a leftover from this wondrous event.
It was an alluring thought, but we now know that Gould’s speculation was wrong. “All of the animals that Gould wrote about can now be related to living phyla. Some of them are on the stem of extant groups, but hadn’t evolved all of their features,” says Mark Purnell, professor of palaeobiology at the University of Leicester. “The idea that the Tully monster could belong to an ancient lineage not closely related to other animal groups is now largely discredited.” So the probability is that it belongs to a known – as opposed to unknown – animal phylum. But which one could that be?
I approached Purnell in 2014, just as research into the Tully monster was kicking back into life. Purnell and his colleague, Sarah Gabbott, had recruited Thomas Clements to study the preservation of Mazon Creek fossils. Very little original organic material has survived in the Mazon Creek fossil nodules. The dead animals were moulded in sediment, decayed and fresh sediment filled the mould and formed a copy of the originals. Apart from its body shape, almost no trace of the original animal remained. This lies at the heart of the conundrum facing palaeobiologists investigating the Tully monster. Fuzzy impressions of its moulded body parts are subject to wildly different interpretations. There are no clearly identifiable anatomical traits in its fossils that are unique to one phylum or another, hence the roadblock as to which of its competing identities is correct.
As Clements and the Leicester team investigated further, they became interested in the Tully monster’s eyes. You might expect that the eyes would decompose quickly but they have microscopic pigments that are very durable in the fossil record, something Gabbott had been studying among other Mazon Creek animals. They suspected that some of the original pigment had survived in the Tully monster. This was potentially the most important breakthrough yet – here at last was hard physical evidence.
The pigment melanin is contained in distinctive granular structures called melanosomes within the eye. Its main job is to absorb stray light that hits the eye to enable a clear image to form. Melanosomes are valuable to palaeontologists because, not only are they durable, but they also form distinctive shapes and layered patterns within the eye cells, depending on the type of animal.
‘Meatball’ and ‘sausage’ shaped melanosomes, stacked in layers, were found. “Only vertebrates have these two types of melanosomes layered together. We are very confident the Tully monster fits within the vertebrates. I think it could be some sort of basal fish,” says Clements.
Clements further points out that, while it had a very unusual anatomy, there are living fish that also possess weird features. Like the Tully monster, the Australian ghost shark has an elongated snout, while the larvae of black dragonfish have their eyes projecting from long stalks. The Tully monster isn’t uniquely odd compared with other vertebrates.
While Clements was studying the eyes, a team headed by Victoria McCoy of Yale University took a fresh look at 1,200 Tully monster specimens. She became interested in what had previously been interpreted as its gut. In other Mazon Creek animals the gut is dark in colour and preserved in three dimensions, whereas in the Tully monster it is light and flattened. However, in the Mazon Creek hagfish Gilpichthys the notochord (a rudimentary backbone) is light and flattened too. Consequently, McCoy re-identified the gut as a notochord – a trait only found in the chordate phylum, to which the vertebrates belong.
Independently, and through completely different methods from Clements, McCoy had reached the same conclusion that the Tully monster was a vertebrate. She went on to identify other anatomical traits that place it within the vertebrates, including a mouth with teeth made of keratin, a tail fin, muscle blocks and gill pouches. A cladistic analysis was also conducted by McCoy, which found that it was a type of lamprey, a primitive jawless fish. Modern lampreys are eel-like parasites that attach their mouths with suckers to the sides of larger fish, where they consume body fluids and rasp off bits of flesh, often causing the death of their unwilling host.
Tully monsters were very different. “It was probably an active predator that hunted by sneaking its proboscis, which may have been expandable, close to its prey and snatching them with its toothy claw. Its eyebar gave it a wide field of vision to aim its clawed proboscis accurately and may have been advantageous in murky, vegetation-choked water. It put in bursts of speed with its tail and probably ate soft-bodied worms or small jellyfish as the teeth in its claw couldn’t crush or bite harder prey. Instead, its tongue extended along its proboscis and rasped off bits of the animal trapped in its claw. We don’t think it was a parasite, like a lamprey, because it doesn’t have a sucking disc to attach to large prey,” says McCoy.
Earlier this year, a team of vertebrate palaeontologists, led by Lauren Sallan of the University of Pennsylvania, roundly rejected these arguments. They argued that its ‘vertebrate’ traits have been misidentified and take issue with nearly every conclusion reached by McCoy’s team.
“The structures identified by McCoy and her team did not read to us as vertebrate. The ‘notochord’ stripe is more likely a gut, and the ‘muscle blocks’ and ‘gill pouch’ segments are too wide, blocky and are not in the correct position to be what they are claimed to be,” says Sallan.
However, she is more cautious about the melanosome evidence. There may be invertebrates with a similar arrangement of melanosomes that have not yet been studied, or it may be a convergent feature that re-evolved as a result of shared genes, she suggests.
The likeliest alternative to the vertebrate identification is that it was a heteropod-like sea snail, as proposed by Foster back in 1979. However, McCoy’s team reject this for a variety of reasons to do with the fossils’ anatomy and preservation. In their view, the evidence fits the lampreys much more convincingly. The other possible identities, such as a type of worm, or an unknown phylum, are too weakly supported by the evidence to carry much favour among scientists.
For a few brief months last year, the identity of the Tully monster seemed to have been resolved, but no longer. For my money, I’m sticking to the vertebrate identity for now. Two large studies independently reached the same conclusion and I think the melanosome evidence, in particular, is hard to challenge.
It is now for the experts to decide whether this enduring mystery can ever be solved.
About the author
James worked in media relations for the British government for many years before following his heart and studying palaeobiology at University College London. He graduated with a first class Master in Science degree in 2002 and has since written numerous features on the evolution of life for New Scientist and other magazines. He can be contacted at: firstname.lastname@example.org.
I am grateful to Bret Beall, Steve Brusatte, Thomas Clements, Sarah Gabbott, Ceinwen Jones, Ted Lawrence, Paul Mayer, Poppy Mathews, Victoria McCoy, Catherine Ogilvie, Mark Purnell, Lauren Sallan and Rosemary Wright for their generous help in preparing this article.
Bibliography and further reading
Beall, B.S. 1991. The Tully monster and a new approach to analyzing problematica.
Clements, T., Dolocan, A., Martin, P., Purnell, M.A., Vinther, J. & Gabbott, S.E. 2016. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature, 532: 500–503. doi:10.1038/nature17647.
Conway-Morris, S. & Simonetta, A.M. (eds). In: The Early Evolution of Metazoa and The Significance of Problematic Taxa (Eds:) 271–286. Cambridge University Press.
Foster, M.W. 1979. A reappraisal of Tullimonstrum gregarium. In: Mazon Creek Fossils (Ed: Nitecki, M.H.) 269-301. Academic Press, New York.
Johnson, R.G., Richardson, E.S. Jr. 1969. Pennsylvanian Invertebrates of the Mazon Creek Area, Illinois: The Morphology and Affinities of Tullimonstrum. Fieldiana Geology, 12 (8): 119–149.
McCoy, V.E., Saupe, E.E., Lamsdell, J.C., Tarhan, L.G., McMahon, S., Lidgard, S., Mayer, P., Whalen, C.D., Soriano, C., Finney, L., Vogt, S., Clark, E.G., Anderson, R.P., Petermann, H., Locatelli E.R. & Briggs, D.E. 2016. The ‘Tully monster’ is a vertebrate. Nature 532: 496–499. doi:10.1038/nature16992.
Richardson, E.S. Jr. 1966. Wormlike Fossil from the Pennsylvanian of Illinois. Science, 151 (3706): 75–76.
Sallan, L., Giles, S., Sansom, R.S., Clarke, J.T., Johanson, Z., Sansom, I.J. & Janvier, P. 2017. The ‘Tully Monster’ is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals. Palaeontology: 1-7. doi:10.1111/pala.12282.
Schram, F.R. 1991. Cladistic analysis of metazoan phyla and the placement of fossil problematica. In: The Early Evolution of Metazoa and The Significance of Problematic Taxa (Eds: Conway-Morris, S. & Simonetta, A.M.) 35–46. Cambridge University Press.